A wide variety of implantable medical devices (IMDs) that deliver a therapy or monitor a physiologic condition of a patient have been clinically implanted or proposed for clinical implantation in patients. Some IMDs may employ one or more elongated electrical leads and/or sensors. Such IMDs may monitor or deliver therapy to the heart, muscles, nerves, the brain, the stomach or other organs. In some cases, IMDs deliver electrical stimulation therapy and/or monitor physiological signals via one or more electrodes or sensor elements, which may be included as part of one or more elongated implantable medical leads. Implantable medical leads may be configured to allow electrodes or sensors to be positioned at desired locations for delivery of stimulation or sensing electrical depolarizations. For example, electrodes or sensors may be located at a distal portion of the lead, and a proximal portion of the lead may be coupled to an IMD housing, which may contain electronic circuitry such as stimulation generation and/or sensing circuitry.
Implantable cardiac devices, such as cardiac pacemakers or implantable cardioverter defibrillators, provide therapeutic stimulation to the heart by delivering electrical therapy signals such as pulses or shocks for pacing, cardioversion or defibrillation via electrodes of one or more implantable leads. In some cases, an implantable cardiac device may sense intrinsic depolarizations of the heart, and control the delivery of therapeutic stimulation to the heart based on the sensing. When an abnormal rhythm of the heart is detected, such as bradycardia, tachycardia or fibrillation, an appropriate electrical therapy (e.g., in the form of pulses) may be delivered to restore the normal rhythm. For example, in some cases, an IMD may deliver pacing, cardioversion or defibrillation signals to the heart of the patient upon detecting ventricular tachycardia, and deliver cardioversion or defibrillation therapy to a patient's heart upon detecting ventricular fibrillation.
Some proposed medical device systems may include a neurostimulator in addition to the implantable cardiac device. As one example, a medical device system may include a spinal cord stimulator and an implantable cardioverter-defibrillator, whereby the spinal cord stimulator may deliver neurostimulation to reduce pain associated with delivery of defibrillation shocks or may provide neurostimulation to assist in regulation of heart rhythms. Examples of such devices include those described in U.S. Pat. No. 5,203,326, issued to Collins eta al., U.S. Pat. No. 5,792,187 issued to Adams, U.S. Pat. No. 5,562,689 issued to Ellsberry, et al. and U.S. Pat. No. 5,994,428 issued to Obel, et al., all of which are incorporated herein by reference in their entireties. Prior art medical devices employing multiple power sources are disclosed in U.S. Pat. No. 6,238,813 issused to Maile, et al., U.S. Pat. No. 5,235,979 issued to Adams and U.S. Pat. No. 4,437,466 issued to Stanley et al., all incorporated herein by reference in their entireties.